This invention relates to radiological well logging methods and apparatus for investigating subsurface earth formations traversed by borehole and, more particularly to gain control means for use in gamma ray energy spectra well logging systems.
It is well known that oil and gas are more likely to be found in commercially recoverable quantities in those earth formations which are relatively porous and permeable than in more highly consolidated formations. It is also well known that an oil or gas filled earth formation or strata may be located by irradiating the earth formations surrounding a borehole with neutrons and measuring the intensity of the resultant gamma radiations which are produced at various levels in the borehole. Various proposals for measuring either the thermal capture gamma ray spectra of such earth formations irradiated with neutrons or the inelastic scattering spectra of the earth formations so irradiated have been proposed in the prior art.
Typical well logging systems proposed in the prior art for measuring these types of gamma ray energy spectra have included neutron generating means and gamma ray detecting means suspended via a well logging cable in the borehole. Surface processing equipment is also usually provided for interpreting electrical pulses produced in response to the gamma rays by the detection means. Usually proportional detectors of the scintillation type have been proposed for measuring the quantity and energy of gamma rays occurring from the resultant neutron irradiation. Electrical pulses whose amplitude is representative of the energy of gamma rays passing through a detector crystal are sent to the surface via the well logging cable. The surface equipment processes these pulses in some manner usually in order to determine the numerical distribution of pulses as a function of their pulse height.
It is apparent that in a system of this sort in which both the pulse height and the number of pulses contains valuable information, that the linearity and repeatability of the system is very important in the accuracy of the resultant measurements. It has been proposed, for example, in a copending application Ser. No. 82,028 filed Oct. 19, 1970, now abandoned, which is assigned to the assignee of the present invention, to utilize a compensating circuit which adjusts the gain of the amplification which takes place at the surface in response to a known amplitude signal generated in the downhole equipment in order to preserve the linearity of the system. This known amplitude signal in the above-mentioned copending application is provided by a downhole pulse generator designed to generate pulses of a desired height, or voltage level, for this purpose.
Gain compensating circuitry such as that disclosed in the above-mentioned copending application has proven to be very useful. The borehole temperature varies nonlinearly as a function of depth. Hence any temperature compensation techniques used in the downhole circuitry or the surface circuitry taken by themselves could be ineffective. Moreover, the effect of the temperature distribution on the cable is unpredictable and the attenuation therefrom can cause an apparent gain drift in the system. In the system described in the above copending application the downhole pulser signal is injected into the system at a point subsequent to the development of the pulse height information by the downhole photomultiplier tube and detector crystal, but before it enters the logging cable. Thus surface or cable generated nonlinearities may be compensated for. Nonlinearities may be introduced into the measurements by the photomultiplier tube itself, however, as opposed to the cable or the other electronics in the system. Also, the detector crystal may introduce nonlinearities into the measurement if its temperature should vary over an extensive range. Nonlinearities introduced from these sources would be uncorrectable.
Accordingly it is an object of the invention to provide a new and improved hydrocarbon indicator well logging system which is less sensitive to changes in gain than previous such systems in the prior art.
Another object of the present invention is to provide improved well logging systems for obtaining the inelastic gamma ray energy distribution due to fast neutron scattering from subsurface earth formations in a manner more accurate than heretofore possible.
The above and other objects, features, and advantages are provided by the present invention which includes methods for generating a gain control signal voltage for controlling the gain of signal processing and analyzing circuitry based on the known position of a particularly recognizable energy peak in the gamma ray energy spectra of the subsurface earth formation.
In the invention downhole neutron generator means is provided together with gamma ray detection means and amplification means for sending resulting electrical pulses to the surface over a well logging cable. At the surface the gamma ray pulses are introduced into a pulse height analyzer apparatus which includes novel gain control circuitry which is pre-set to observe a particular known energy peak or feature occurring in the gamma ray spectra of the subsurface earth formations. In a particular embodiment described in more detail herein the 2.22 MEV hydrogen gamma ray energy spectrum peak is used for this purpose. After an initial set up, the hydrogen peak is centered in an energy range which is monitored by the gain control circuitry. Any nonlinearity or drift in the gain of the system is compensated for by the gain control circuitry which develops an error signal proportional in magnitude and whose algebraic sign is proportional to the direction of the drift. This error or gain control voltage is applied in an inverse feedback manner to compensate and maintain linearity of the amplification of the system. In this manner the known energy hydrogen peak is kept in the energy window being monitored.
The foregoing and other objects, features and advantages of the invention will appear more fully hereinafter. The detailed description which follows together with the accompanying drawings illustrate an embodiment of the present invention. It is to be understood, however, that the drawings and descriptions herein are for illustrative purposes only and are not to be construed as defining the limits of the invention.